Interestingly, this doesn't work for me. OPTION+LEFT still gives [D, OPTION+RIGHT still gives [C. Are there any other possible sources of conflict beyond the general keyboard settings and profile keys?

@jaydeesigns sorryfor leaving this reply late — writing this here for posterity in case anyone else faces the same problem.

Yup, one of the tradeoffs is you have to obscure whatever useful parts of the input are there too. It's worth noting that feedback on the selected file is not at all standardized, but you can extract whatever the browser will allow using the following Javascript (where fileInput is a reference to the file input):

fileInput.value;

If I'd selected the file at D:\some\thing\foo.bar the output would be:

Note I didn't use the change event above because IE will only fire that once the user has navigated (tabbed or clicked) away from the input after using it. Instead we listen for the click event (which will trigger on keyboard activation too), and use a setTimeout to make the function resolve as soon as possible (zero milliseconds): in this way, the user clicks, the dialogue has time to open, and Javascript only executes when the user has finished inputting.